The invention relates to a circuit for providing a controlled resistance. In particular, the circuit is adapted to simulate the characteristic of a resistance-variable sensing element, particularly a platinum temperature detecting element.
It is known to employ an electrical circuit to provide a controlled resistance, e.g. by use of a resistance bridge. However, such known circuits suffer from the disadvantage that it is difficult to provide an accurately controlled resistance which is stable over a long period of time and over a range of ambient temperatures. Furthermore, the known circuits require adjustment in order to provide a desired controlled resistance and cannot automatically produce any selected value of resistance.
The use of temperature detectors which depend on the measurement of variation of the resistance of an element with temperature is also well known. For accurate measurement in the range -200 to +600 deg C., the element is most commonly a grade of pure platinum which is referred to as Pt100. Elements of Pt100 are manufactured and used in accordance with national standards such as British Standard (BS) 1904:1984. The standards define the resistance of the element over the temperature range and the tolerances permitted at fixed temperature points for different classes of measurement. Standardised Pt100 elements are highly stable and are interchangeable but the resistance-temperature characteristic is only approximately linear, with a slope of about 0.4 ohm/deg C, and variation in slope prevents extrapolation over any extended range. Calibration of a measuring instrument therefore requires comparison at a large number of points on the temperature scale.
For each point on the scale it is usual to connect to the instrument a resistance of exactly known value which corresponds to that of a Pt100 element at the same temperature. This process is known as simulation since it replaces the use of an actual element immersed in a succession of constant temperature baths or enclosures. The problem of calibration is enhanced because Pt100 thermometry is not only a laboratory technique but is used in the routine supervision of industrial processes. This has been made possible by the availability of compact microelectronic measuring instruments at low cost. The long-term stability of such instruments may be uncertain and some convenient means of recalibration in the field is very desirable.
Calibration requires an excitation current to be passed through the simulator resistance and the instrument is most commonly some form of bridge. Alternatively the instrument may provide a source of constant current and means for measuring the voltage drop on the resistance. The simulator resistance itself is available in the form of a resistance box in which a number of tappings directly identify the corresponding temperatures. For intermediate points the user must resort to a conventional switched decade resistance box, which essentially restricts the procedure to laboratory conditions. An approach to continuity of calibration over the temperature range has been made by the use of a high-resolution multi-turn potentiometer but extreme care is required in manual setting.